Hey West,
Waay long time no see. It appears you've gotten a might taller. (and without my permission I might add.)
Ran into a similar situation with two of my nephews.
Option 1: [Total Paranoia, I.E. The Crazy Boyfriend] Will rob you of your sleep and your sanity. In this option, you can camera the crap out of everything, escort her to and from work, go armed, and basically wear a "don't mess with me or I will rip your face off." look everywhere you go. Expensive, maintenance heavy, and most likely to lead to a confrontation. Helps to be over 6ft tall and 180+ lbs
Option 2: [Go Dark, referenced earlier] Change phone #, do not give the new number out, even to old friends, as they might share with the person stalking her. Turn off Facebook, change email, reset security on all manner of things, but especially social media. In extreme cases, move away, change name, etc. - Note, this is like a "witness protection" program, it wont work if she isn't committed.
Option 3: [The Direct Approach: usually a REALLY bad idea] Pick a fight with the guy, incite a confrontation, beat the whoohah out of him. - Did I mention really bad idea?
Option 4: [Re-evaluate the relationship] Self reflection, and many HARD questions need to be asked (TO YOURSELF). "Is this worth it?" comes to mind. Yes, you obviously care for her. However, how many boyfriends in how long a period of time? She has obviously taken and shared nudes in the past, is this (or similar) behavior likely to change? What made her a target for this sort of blackmail / stalking in the first place? Being beautiful doesn't truly enter into it, there are plenty of girls (and guys) that are beautiful and useless. As my dad used to say: "You can't shake a hooker tree and get a housewife." Is it her self esteem, or yours that is being fed by this relationship?
All possible choices. In my own case, 2 years into our marriage, (been 10 years now) a guy we knew started following my wife around regularly. Like all over town. My wife called me and I showed up at the store she was at, and there he was. I walked up to him, directly told him that I knew what he was up to, and that he should leave her alone, now, and never follow her again. I said it very quietly, but firmly, without an ounce of threat in my voice. It was enough. Now my wife says that because I'm such a gregarious person normally, when I'm super quiet and focused people don't know what to do or how to react. Some people can be intimidated, some can't, but most people fear uncertainty.
Best of luck in your decision...we can advise, but it is your decision. Your grown, and your going to have to figure it out like all of us did.
-Dan